The invention relates to gateless thyristors (for unidirectional protection) or gateless triacs (for bidirectional protection), that is, structures including four semiconductor layers of alternate NPNP conductivity types. In these gateless components, conduction occurs because of the breakdown of the junction between the central layers of the structure. This avalanche mode generally occurs first in a localized area then progressively extends throughout the component.
As shown in FIG. 1A, such a component is essentially formed by superimposed semiconductor layers of alternate conductivity types which are, respectively, between a first main surface and a second main surface: an N-type cathode layer N1, a P-type gate layer P1, a middle N-type layer N2, and a P-type anode layer P2. The cathode and anode layers are respectively coated with metallizations M1 and M2. Generally, the structure includes "shorting" holes CC distributed beneath the cathode metallization. Through these holes, gate layer P1 crosses the cathode layer N1 to the cathode metallization. These shorting holes are sometimes called emitter shorts.
FIGS. 1B and 1C are exemplary top views of the component of FIG. 1A in the absence of metallization M1 to better illustrate shorting holes CC. These figures show two conventional shorting hole arrangements, disposed so as to form respectively a square and a triangle, or an oblique pattern. Characteristic features of these holes are the distance D between two adjacent holes (assuming that all the holes are equidistant) and the diameter d of each hole (assuming that all the holes have an equal diameter).
Those skilled in the art can, from a unidirectional protection component of the gateless thyristor-type, such as illustrated in FIG. 1A, form a bidirectional protection component of the gateless triac-type by disposing in a single monolithic structure two reversely connected gateless thyristors of the type shown in FIG. 1A.
Furthermore, it will be noted that FIGS. 1A, 1B and 1C are partial views and do not show the component edges for which conventional processing steps are required to have a suitable breakover voltage.
FIGS. 2 and 3 are characteristic curves of a conventional protection component of the gateless thyristor-type. These curves correspond to unidirectional components. The characteristic curves of bidirectional components can be immediately deduced since they are symmetrical with respect to the origin.
FIG. 2 shows the curve of current I as a function of voltage V of a conventional gateless thyristor. As long as the applied voltage remains lower than a threshold value V.sub.BR, the device is in a blocking mode and no current flows. When overvoltage occurs and voltage increases from value V.sub.BR and approaches value V.sub.B0, current starts flowing in the component in accordance with a characteristic curve A. When the applied voltage exceeds V.sub.B0, the characteristic curve becomes curve B and the operation point is fixed to a low voltage value V.sub.ON and to a current I.sub.ON which is determined by the characteristics of the power supply and of the circuit. When the overvoltage disappears, current in the component decreases down to a hold current value I.sub.H. When the current becomes lower than I.sub.H, the component returns to the blocking mode.
The characteristic curve illustrated in FIG. 2 exemplifies static phenomena. FIG. 3 shows the shape of current I and voltage V as a function of time in a component of the gateless thyristor-type subjected to a voltage surge. The voltage very rapidly goes from a substantially zero voltage to V.sub.BR. Then, dynamic phenomena cause the voltage to increase beyond value V.sub.BR before decreasing again to this value then to value V.sub.ON. The current increase occurs with an initial substantially linear slope, dI/dt. Although the current and voltage curves are superimposed, they are not drawn to the same time scale. For example, value V.sub.B0 is reached within a time period lower than 1 microsecond, while time t.sub.1, at the end of which the current reaches a maximum value IPP, can be approximately 10 microseconds, and time t.sub.2, at the end of which the current has decreased to value IPP/2, can be approximately 100 microseconds.
The major characteristics of a protection component are:
the difference between values V.sub.BR and V.sub.B0 determines the triggering precision of the device; PA1 the dynamic surge over value V.sub.BR must be as low as possible; PA1 the value of dI/dt that the component can withstand must be as high as possible; that is, the component must be capable of withstanding high current densities while switching on; PA1 the value of the hold current I.sub.H which must be relatively high. In practice, in a protection component, it is desired, once overvoltage has disappeared, that the circuit coupled to the protection component resumes operating as soon as possible. If the value of the hold current I.sub.H is too low, the protection component remains conductive when the overvoltage has disappeared.
It is known that the shorting hole density affects these various parameters. If this density increases, one obtains an advantageous effect, namely, the hold current I.sub.H increases, and adverse effects, namely, the static and dynamic values of V.sub.B0 increase and the value of dI/dt that the component can withstand decreases.
Thus, in the prior art, mean density values of the shorting holes are selected so as to provide a satisfactory compromise between these various effects.
In thyristor or triac structures including a control gate, the density of the shorting holes has been modified at the vicinity of the gate area.